Precipitation Changes

The Southwest is particularly vulnerable to climate change because of its aridity, and the region is projected to become even drier in decades to come.

Temperature rise alone increases the risk of regional drought, with its effect on evaporation rates and extreme events. On top of that, precipitation is projected to decline in the region—even as the risk of heavy rainfall events increases. This page reviews:

Figure 1. Using an ensemble of 18 global climate models and the moderate A1B emissions scenario, researchers at the NOAA Earth System Research Laboratory (ESRL) predict a reduction in precipitation across the Southwest by the end of the century.
| Enlarge This Figure |Credit: Jeremy Weiss, The University of Arizona

Projections for more drought

Precipitation is projected to drop by 5 percent by century’s end (relative to average precipitation over the last three decades of the 20th century) for much of Arizona and New Mexico, based on results from an ensemble of 18 global climate models (Figure 1). A 10 percent decline could be in store for the southern half of Arizona, while northeastern New Mexico is projected to remain roughly stable, based on these estimates.

Such a decrease could have a more serious impact than the numbers suggest. The decrease of water draining from the landscape into rivers and reservoirs typically can be double or triple the proportional reductions in rainfall amounts, especially when combined with higher temperatures.1

In an analysis that looked at the U.S. Southwest, Richard Seager and colleagues found the 19 models used in their assessment were converging upon an increase in aridity for this region.2 For their study, they defined the Southwest as the land area stretching east-west from Houston to San Francisco and north-south from Denver to Monterrey, Mexico.

Their Science paper considered the joint effect of increased evaporation in summer and declining precipitation in winter and summer. Considering these two factors in tandem, the models projected an average moisture loss amounting to about a third of an inch of rainfall per month across the region. The analysis did not specify values by areas within the region.

Precipitation changes remain much more difficult to predict than temperature because precipitation is more variable and operates at smaller scales. Both factors hamper prediction. When comparing Global Climate Model (GCM) simulations of climate to what actually occurred, researchers found the results roughly matched 50 to 60 percent of the time for precipitation. This compares to about 95 percent of the time for temperature.3

Figure 2. The Hadley cell circulation illustrates how rising air in the superheated tropics descends in the subtropics. This creates high-pressure zones in subtropical regions, including the U.S. Southwest.
| Enlarge This Figure |Credit: Barbara Summey, NASA Goddard Visualization Lab

Despite the challenges in predicting precipitation, the overall agreement for increased aridity among models indeed seems to foretell of dry times ahead for the region. Only one of the 19 GCMs evaluated by Seager and his team suggested a potential decrease in aridity for the southwestern quadrant of the country.

One suggested mechanism behind the projection for reduced precipitation relates to a global atmospheric pattern known as Hadley Cell circulation. The area under the Hadley Cell’s descending air is projected to widen in years to come. As a result, the jet stream that transports rain and snow during spring and winter is expected to move toward the North Pole (Figure 2). Thus winter storms could enter the western United States in a more northerly position, bypassing the Southwest more often than it currently does.

In terms of summer rainfall, many GCMs project a northward shift of the subtropical anticyclone—the climate pattern that helps set up the monsoon (Figure 3). This could decrease annual precipitation in the U.S. Southwest and northern Mexico, by some interpretations.4 Other factors, however, suggest the potential for an increase in monsoon rainfall.5Summer rains typically operate at smaller scales than winter rainfall, making them more challenging to predict.

Figure 3. The air circulation patterns at 18,000 feet show the signature “anticyclone” that helps define the North American monsoon.
| Enlarge This Figure |Credit: Jeremy Weiss, The University of Arizona

Meanwhile, hotter temperatures are likely to bring higher evaporation rates, much as they do during summer compared to winter. As a result, dry spells between rains can have more severe impacts on the landscape, especially in spring and summer.

Projections for more floods

While the region is expected to dry out, it paradoxically is likely to see larger, more destructive flooding. This relates in part to the well-documented physical law that warm air holds more water vapor than cooler air. Atmospheric physicists calculate that the moisture-holding capacity of modern air increases by roughly 4 percent for every rise of one degree F. Thus, climate models project a future increase in atmospheric water vapor along with the increase in global temperature.

In fact, this projection was called “perhaps the single most robust aspect of global warming simulations” by scientists comparing modern GCMs.6

In effect, the models are agreeing that the air will hold more water vapor as it warms up. This creates conditions that potentially could lead to bigger and more frequent floods by causing more intense, heavy rainfall events, such as hurricanes and tropical storms7.

Arizona and New Mexico typically receive 10 percent or more of their annual precipitation from storms that start out as tropical cyclones in the Pacific Ocean.5 Some of the largest floods in the Southwest have occurred when a remnant tropical storm hit a frontal storm from the north or northwest, providing energy to empower a remnant tropical storm.8

Observations

The observational evidence shows some support for the projections for a poleward shift in the jet stream, a pattern that could mean El Niño events might often fail to bring rain and snow to the Southwest. In practice, that pattern might look a lot like the winter of 2006–07, when Denver received record snowfall while Arizona’s dry winter pushed much of the state back into drought.

The subtropical jet stream, which tracks moisture across the U.S., tends to be located at the northern end of the descending Hadley Cell. Other research teams have measured changes in the atmosphere that suggest the Northern Hemisphere Hadley Circulation is shifting further north.10

In recent decades, the jet stream has maintained a pattern linked to drier southwestern springs. University of Arizona researchers Stephanie McAfee and Joellen Russell documented that drier spring seasons (February–April) occurred in much of the region west of the Rocky Mountains between 1978 and 1998.9

They found lower precipitation rates in the Southwest correlated with positive phases of the atmospheric pattern known as the Northern Annular Mode (NAM), which they attributed to an associated northward shift of the jet stream during these years. Meanwhile, the Rocky Mountains, headwaters of the Colorado and Rio Grande rivers, tended to register higher precipitation rates during these positive phases, consistent with the observation that the jet stream was shifting north.

Outside of spring, trends are more difficult to detect. Yearly rainfall remains highly variable in the Southwest, with no detectable increase or decrease in New Mexico or Arizona since the 1930s. Since 1900, a trend toward decreasing precipitation affected western and northern Arizona, while other parts of the Southwest registered a slight increase.11

When considered by climate division, overall precipitation has actually increased in many Arizona and New Mexico climate divisions from 1950 to 2003. As discussed above, this is the opposite trend of what might be expected given projections for future climate in the greater Southwest.

Precipitation records contain a high degree of variability, however, as they are influenced by a variety of other factors in addition to climate change. For instance, much of the upward trend in the precipitation record from 1950 probably stemmed from an abundance of El Niño events in the second half of the record, according to Martin Hoerling, the researcher who conducted the analysis.12

Also, the starting date can make a big difference for precipitation records. In this case, large swaths of Arizona and New Mexico were going through an extended drought during the 1950s. Precipitation levels were at least as low during the 1950s drought as they were through the early 2000s. Even so, warmer temperatures contributed to making certain impacts of the modern drought more severe than the 1950s drought.13

Drought has been a recurring theme in the Southwest, based on records that go beyond the instrumental record as well. Tree-ring evidence shows that one of the most widespread and lengthy droughts in the region occurred during the Medieval Warm Period.14

Even so, the projection for the globe as a whole for more extreme precipitation events also applies to the Southwest. One analysis included the Southwest in the U.S. area featuring heavier rains this past century, even as many southwestern areas registered drier soils. Higher temperatures are taking their toll.11

In short, both projections and observations indicate residents of the arid Southwest can count on more extremes in years to come. Along with an overall increase in aridity, extreme variability is likely to become the future norm.